1. Field of the Invention
The present invention relates to a method for inspecting a projection exposure apparatus used for manufacturing a semiconductor device, and particularly to a method for inspecting performance of a projection optical system of an exposure apparatus.
2. Description of the Related Art
A lithography technique is generally used to manufacture a circuit pattern of a semiconductor device. In a projection exposure apparatus used in a lithography process, light emitted from an illumination optical system enters a photomask on which a circuit pattern is drawn. Light passing through the photomask is converged by a projection optical system. Further, in general cases, the circuit pattern of the photomask is focused and projected on a substrate applied with a photosensitive material, e.g., a silicon wafer applied with photoresist.
Recently, as the semiconductor device pattern to be formed is downsized, the dimension of the pattern to be formed by the optical lithography becomes severer.
In case of the exposure apparatus, as the pattern of the semiconductor device in comparison with the exposure wavelength is shrunk more, diffraction of light becomes more remarkable. Also, it is known that the diffraction angle increases as the period of the pattern decreases. To form a micro pattern, the diffraction light propagating in the direction in which it goes away from the optical axis needs to be captured and converged onto the wafer. Therefore, the diameter of the projection optical system needs to be increased in order to form a more microscopic pattern. In other words, the numerical aperture NA of a projection optical system needs to be increased. In case of the exposure using a photomask which has a one-dimensional periodic pattern such as line & space pattern, a plurality of discrete diffraction light are occurred. The discrete diffraction light are straight zeroth-order diffraction light, first-order up to higher-order diffraction light which have predetermined diffraction angle. In order to form a one-dimensional periodic pattern on the wafer, first-order diffraction light needs to be captured and converged with zeroth-order diffraction light.
Meanwhile, if the projection lens forming part of the projection optical system becomes large, a problem occurs in that light transmittance depending on the light path changes. In case of exposing a relatively large pattern with respect to the exposure wavelength, the light diffraction angle is small. In this case, only the portion of light that passes through the optical axis of the projection lens contributes to focusing of an image. That is, the paths of zeroth-order diffraction light and first-order diffraction light that are used for focusing an image are different from each other. Accordingly, the intensity of each diffraction light is not influenced by changes of the transmittance of the projection optical system.
In contrast, the diffraction angle is large in case of exposing a micro pattern, and therefore, zeroth-order diffraction light and first-order diffraction light are different from each other. Accordingly, if the transmittance in the projection optical system changes depending on light paths, diffraction light which reaches a wafer is influenced by changes of the transmittance, and as a result, the intensity of each diffraction light changes.
In conjunction of design of the projection optical system, changes of the transmittance depending on the light paths are not caused. But in practice, the drawbacks can be occurred imperfect anti-reflection coating on lens surface, light absorption of lens material, and the like. However, proposals have not yet been made for a method of directly measuring this phenomenon without disassembling the exposure apparatus.
A transmittance change depending on the light paths causes the intensities of zeroth-order diffraction light and first-order diffraction light to change. Since photoresist pattern on a wafer is formed by interference between these diffraction lights, a change of the intensities influences the pattern image focusing performance. As a result of this, it is considered that the micro pattern transferring performance of the projection optical system is deteriorated.
If a micro periodic pattern is formed by interference between zeroth-order diffraction light and first-order diffraction light, light generated by the interference constructs a bright part and a dark part. The degree of brightness is expressed as an amount of contrast. If bright and dark parts are clearly distinguished from each other, it is called “high contrast”. The higher the contrast of interference light, the easier the transfer of the pattern onto the wafer. In other words, the contrast should desirably be high in order to widen the focus margin and the exposure dose margin. The contrast is determined by amplitude and phases of lights which interference each other.
If a circuit pattern is designed supposing that drawbacks described above do not occur, the contrast of interference light formed on the wafer is rendered insufficiently high. As a result, no pattern may be formed. At present, shrinkage of patterns has progressed and lithography design using simulations has come to have a significant meaning. It is undesirable that unexpected drawbacks of this kind occur in the exposure apparatus. In the process of assembling an exposure apparatus, drawbacks should be removed or extents of drawbacks should previously measured and which then have to be taken into consideration in case of estimate and designing of to-be-formed pattern based on exposure simulations.
An example of measurement of contrast, which has been conventionally carried out, will be explained with reference to FIG. 1. FIG. 1 shows relationship between formed photoresist patterns (left side) and relative light intensities I (=1/D) (right side). The contrast is expressed by the following expression with use of a light intensity I1 at peaks of light intensity and a value I5 at a minimum light intensity between peaks.(contrast)=(I1−I5)/(I1+I5)=(1/D1−1/D5)/(1/D1+1/D5)=(D5−D1)/(D5+D1)
In the expression, the intensity I5 at which the light intensity comes to peaks is an intensity of the minimum between peaks of light intensities. Although presence or absence of reduction of the contrast can be confirmed by the method shown in FIG. 1, factors which cause reduction of the contrast is very difficult to specify.
Another phenomenon which is caused by a change of the diffraction intensity is a positional shift of pattern depending on focusing onto a wafer, which is caused by the gravity center of the intensity of the diffraction light shifts from the center of the projection optical system. Where a line-and-space pattern are cited as an example, two of positive and negative first-order diffraction lights are generated with a center of zeroth-order diffraction light taken as a symmetry point. If there is a difference between intensities of the positive and negative first-order diffraction lights, the position where the pattern is formed shifts depending on the defocus amount of the wafer.
The shift of the position where the pattern is formed depending on the defocus amount of the wafer is occurred due to factors other than a transmittance change depending on the light paths, such as coma aberration or illumination telecentricity error. Therefore, it is difficult to specify the factor which causes the shift of the position only by the measurement of the relationship between the defocus and misalignment of the pattern.